Data transmission apparatus, data receiving apparatus and method executed thereof

ABSTRACT

A data transmitting apparatus  1101  generates, by using predetermined first key information  11;  and information data  10,  a multi-level signal  13  in which a signal level changes so as to be approximately random numbers, and converts the multi-level signal  13  into a modulated signal  14,  in a predetermined modulation method and transfer the same. A data receiving apparatus  1201  demodulates the modulated signal  14  so as to be converted into a multi-level signal  15,  and reproduces information data  18  from the multi-level signal  15,  using second key information  16  which has the same content as first key information  11  used by the data transmitting apparatus  1101.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatuses for performing ciphercommunication in order to prevent illegal eavesdropping and interceptionby a third party, more particularly, relates to a data transmittingapparatus, a data receiving apparatus, and a method executed thereby forperforming data communication through selecting and setting a specificencoding/decoding (modulating/demodulating) method between a legitimatetransmitter and a legitimate receiver.

2. Description of the Background Art

Conventionally, in order to perform communication between specificparties, there has been adopted a structure for realizing secretcommunication by sharing key information for encoding/decoding betweentransmitting and receiving ends, and by performing, based on the keyinformation, an operation/inverse operation on information data (plaintext) to be transmitted, in a mathematical manner. FIG. 11 shows aconfiguration of a conventional data communication apparatus based onthe above-described structure.

In FIG. 11, the conventional data communication apparatus has aconfiguration in which a data transmitting apparatus 9001 and a datareceiving apparatus 9002 are connected to each other via a transmissionline 913. The data transmitting apparatus 9001 includes an encodingsection 911 and a modulator section 912. The data receiving apparatus9002 includes a demodulator section 914 and a decoding section 915.

In the data transmitting apparatus 9001, information data 90 and firstkey information 91 are inputted to the encoding section 911. Theencoding section 911 encodes (encrypts), based on the first keyinformation 91, the information data 90. The modulator section 912converts the information data encrypted by the encoding section 911 intoa modulated signal 94 in a predetermined modulation method and transmitsthe same to the transmission line 913.

In the data receiving apparatus 9002, the demodulator section 914demodulates, in a predetermined demodulation method, the modulatedsignal 94 transmitted via the transmission line 913. To the decodingsection 915, second key information 96 which has the same content as thefirst key information 91, which is shared with the encoding section 911,is inputted. The decoding section 915 decodes (decrypts) the modulatedsignal 94 in accordance with the second key information 96 and outputsthe original information data 90.

Here, by using an eavesdropper's data receiving apparatus 9003,eavesdropping by a third party will be described. In FIG. 11, theeavesdropper's data receiving apparatus 9003 includes an eavesdropper'sdemodulator section 916 and an eavesdropper's decoding section 917. Theeavesdropper's demodulator section 916 eavesdrops on the modulatedsignal 94 transmitted between the data transmitting apparatus 9001 andthe data receiving apparatus 9002, and decodes the eavesdroppedmodulated signal 94 in a predetermined demodulation method. Theeavesdropper's decoding section 917 attempts decoding of the demodulatedinformation data, in accordance with third key information 99. Here, dueto no key information sharing with the encoding section 911, the thirdkey information 99 is different in content from the first keyinformation 91. Therefore, the eavesdropper's decoding section 917cannot accurately reproduce the original information data 90 inputted tothe encoding section 911 even if the decoding is performed based on thethird key information 99.

A mathematical encryption (or also referred to as a computationalencryption or a software encryption) technique based on suchmathematical operation may be applicable to an access system asdescribed, for example, in Japanese Laid-Open Patent Publication No.9-205420 (hereinafter referred to as patent document 1). That is, In aPON (Passive Optical Network) structure in which an optical signaltransmitted from an optical transmitter is divided by an opticalcoupler, and distributed to optical receivers at a plurality of opticalsubscribers' houses, such optical signals that are not desired and aimedat other subscribers are inputted to each of the optical receivers.Therefore, information data for each of the subscribers is encrypted byusing key information which is different by the subscribers, whereby itis possible to prevent a leakage/eavesdropping of mutual information andrealize safe data communication.

However, in the case of the conventional data communication apparatusbased on the mathematical encryption technique, even if the eavesdropperdoes not share the key information, it is theoretically possible for theeavesdropper to succeed in decryption, with respect to a cipher text(modulated signal or encrypted information data), by means of anoperations using all possible combinations of key information (anall-possible attack), or by means of a special analysis algorithm.Particularly, improvement in processing speed of a computer has beenremarkable in recent years, and thus there is a problem in that if a newcomputer based on a novel principle such as a quantum computer isrealized in the future, it is possible to eavesdrop on the cipher texteasily within finite lengths of time.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a datatransmitting apparatus and a data receiving apparatus which cause aneavesdropper to take a significantly increased time to analyze a ciphertext and consequently realize highly concealable data communication.

The present invention is directed to a data transmitting apparatus and amethod for performing cipher communication. To attain the above object,the data transmitting apparatus of the present invention includes amulti-level code generation section, a combining section, anerror-correction encoding section, a multi-level processing section, anda modulator section. The data transmitting method of the presentinvention is realized by executing respective steps included in themethod.

The multi-level code generation section generates, by usingpredetermined key information, a multi-level code sequence in which asignal level changes so as to be approximately random numbers. Thecombining section combines accompanying data to the multi-level codesequence with predetermined frequency and generates a combinedmulti-level signal. The error-correction encoding section adds anerror-correction code to information data, in accordance withpredetermined error correction encoding processing, and outputserror-correction encoded information data. The multi-level processingsection combines the combined multi-level signal and theerror-correction encoded information data in accordance withpredetermined processing, and generates a multi-level signal having alevel uniquely corresponding to a combination of levels of the combinedsignals. The modulator section generates a modulated signal, in apredetermined modulation method, based on the multi-level signal.

The accompanying data is a synchronous signal which is synchronizingwith the information data, an N frequency-dividing clock of theinformation data, or a predetermined code-synchronous pattern.

Further, the present invention is also directed to a data receivingapparatus and a method for performing cipher communication. To attainthe above object, the data receiving apparatus of the present inventionincludes a demodulator section, a multi-level code generation section, amulti-level identification section, and an error-correction decodingsection. The data receiving method of the present invention is realizedby executing respective steps included in the method.

The demodulator section demodulates a modulated signal, in apredetermined modulation method, generated based on error-correctionencoded information data and a combined multi-level signal havingaccompanying data included therein, and outputs a multi-level signalobtained by the demodulation. The multi-level code generation sectiongenerates, by using predetermined key information, a multi-level codesequence in which a signal level changes so as to be approximatelyrandom numbers. The multi-level identification section identifies themulti-level signal in accordance with the multi-level code sequence, andoutputs data which is identified and reproduced. The error-correctiondecoding section detects, from the data reproduced by the multi-levelidentification section, difference between the combined multi-levelsignal and the multi-level code sequence, in accordance withpredetermined error-correction decoding processing, and outputs a resultof the detection as the accompanying data, and also outputs informationwhich is error-correction decoded as the information data.

Here, in the case where the multi-level code generation sectiongenerates, by using predetermined key information and a synchronoussignal, a multi-level code sequence in which a signal level changes soas to be approximately random numbers, it is possible to further includea synchronous extraction section for inputting the accompanying data,extracting a synchronous signal synchronizing with the accompanying datain accordance with a predetermined procedure, and outputting theextracted synchronous signal to the multi-level code generation section.

It is preferable that the synchronous extraction section inputs apredetermined code-synchronizing pattern and accompanying data, extractsthe synchronous signal synchronizing with the accompanying data inaccordance with a predetermined procedure, and outputs the extractedsynchronous signal to the multi-level code generation section.

According to the present invention, the information data isencoded/modulated into the multi-level signal by using the keyinformation, and the received multi-level signal is decoded/demodulated,by using a common key information, whereby a signal-to-noise power ratiois adjusted appropriately. Accordingly, time for analyzing a cipher textis increased significantly, whereby it is possible to perform highlyconcealable data communication.

Further, the error-correction code is added to the information data tobe transmitted, and the multi-level code sequences respectively in thedata transmitting apparatus and the data receiving apparatus are causedto be in discord with each other by using the accompanying data, wherebythe data to be transmitted is encrypted, and an error of the receivingdata occurring in the receiving apparatus is corrected. Accordingly, theinformation data and the accompanying data is transmitted/receivedsimultaneously, whereby it is possible to provide a highly concealabledata communication apparatus.

Further, the synchronous signal is extracted from the accompanying data,and multi-level signal is synchronized with and identified by themulti-level code sequence generated based on the synchronous signal,whereby it is possible to realize a data communication apparatus of asimple configuration having a synchronous system.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of data communicationapparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating waveforms of a transmission signal ofthe data communication apparatus according to the first embodiment ofthe present invention.

FIG. 3 is a diagram illustrating names of the waveforms of thetransmission signal of the data communication apparatus according to thefirst embodiment of the present invention.

FIG. 4 is a diagram illustrating quality of the transmission signal ofthe data communication apparatus according to the first embodiment ofthe present invention.

FIG. 5 is a block diagram showing a configuration of the datacommunication apparatus according to the second embodiment of thepresent invention.

FIG. 6 is a flowchart illustrating a method executed by a datatransmitting apparatus 1102 of FIG. 5.

FIG. 7 is a flowchart illustrating a method executed by a data receivingapparatus 1202 of FIG. 5.

FIG. 8 is a diagram showing an action (legitimate communication) of thedata communication apparatus according to the second embodiment of thepresent invention.

FIG. 9 is a diagram showing an action (eavesdropping) of the datacommunication apparatus according to the second embodiment of thepresent invention.

FIG. 10 is a block diagram showing a configuration of the datacommunication apparatus according to the third embodiment of the presentinvention.

FIG. 11 is a block diagram showing a configuration of a conventionaldata communication apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein after, embodiments of the present invention will be describedwith reference to drawings.

First Embodiment

FIG. 1 is a block diagram showing a configuration of data communicationapparatus according to a first embodiment of the present invention. InFIG. 1, the data communication apparatus according to the firstembodiment has a configuration in which a data transmitting apparatus1101 and a data receiving apparatus 1201 are connected to each other viaa transmission line 110. The data transmitting apparatus 1101 includes amulti-level encoding section 111 and a modulator section 112. Themulti-level encoding section 111 includes a first multi-level codegeneration section 111 a and a multi-level processing section 111 b. Thedata receiving apparatus 1201 includes a demodulator section 211 and amulti-level decoding section 212. The multi-level decoding section 212includes a second multi-level code generation section 212 a and amulti-level identification section 212 b. As the transmission line 110,a metal line such as a LAN cable or a coaxial line, or an opticalwaveguide such as an optical-fiber cable can be used. Further thetransmission line 110 is not limited to a wired cable such as the LANcable, but can be free space which enables a wireless signal to betransmitted.

FIG. 2 and FIG. 3 are diagrams illustrating waveforms of a modulatedsignal 14 outputted from the modulator section 112. Hereinafter, withreference to FIG. 1 to FIG. 4, actions of the data communicationapparatus according to the first embodiment will be described.

The first multi-level code generation section 111 a generates, by usingpredetermined first key information 11, a multi-level code sequence 12((b) of FIG. 2) in which a signal level changes so as to beapproximately random numbers. The multi-level code sequence 12 ((b) ofFIG. 2) and information data 10 ((a) of FIG. 2) are inputted to themulti-level processing section 111 b. The multi-level processing section111 b combines the multi-level code sequence 12 and the information data10 in accordance with a predetermined procedure, and generates amulti-level signal 13 ((c) of FIG. 2) having a level corresponding to acombination of the both signal levels. For example, in the case wherethe level of the multi-level code sequence 12 changes to C1/C5/C3/C4with respect to time slots t1/t2/t3/t4, the multi-level processingsection 111 b regards the level of the multi-level code sequence 12 as abias level, adds the information data 10 to the multi-level codesequence 12, and then generates the multi-level signal 13 in which asignal level changes to L1/L8/L6/L4. The modulator section 112 modulatesthe multi-level signal 13 in a predetermined modulation method into themodulated signal 14 and transmits the same to the transmission line 110.

Here, as shown in FIG. 3, an amplitude of the information data 10 isreferred to as an “information amplitude”, a total amplitude of themulti-level signal 13 is referred to as a “multi-level signalamplitude”, pairs of levels (L1, L4)/(L2, L5)/(L3, L6)/(L4, L7)/(L5, L8)which the multi-level signal 13 may take corresponding to levelsc1/c2/c3/c4/c5 of the multi-level code sequence 12 are respectivelyreferred to as a first to a fifth “bases”, and a minimum intervalbetween the signal levels of the multi-level signal 13 is referred to asa “step width”.

The demodulator section 211 demodulates the modulated signal 14transmitted via the transmission line 110, and reproduces a multi-levelsignal 15. The second multi-level code generation section 212 apreviously shares second key information 16 which has the same contentas the first key information 11, and, generates, based on the second keyinformation 16, a multi-level code sequence 17. The multi-levelidentification section 212 b identifies (binary determination) themulti-level signal 15 by using the multi-level code sequence 17 as athreshold, and reproduces information data 18. Here, the modulatedsignal 14, in the predetermined modulation method, which istransmitted/received between the modulator section 112 and thedemodulator section 211 via the transmission line 110, is obtained bymodulating an electromagnetic wave (electromagnetic field) or an opticalwave using the multi-level signal 13.

Note that the multi-level processing section 111 b may generate themulti-level signal 13 by using any methods, in addition to a method ofgenerating the multi-level signal 13 by adding up the information data10 and the multi-level code sequence 12 as above described. For example,the multi-level processing section 111 b may generate, based on theinformation data 10, the multi-level signal 13, by modulating anamplitude of the level of the multi-level code sequence 12.Alternatively, the multi-level processing section 111 b may generate themulti-level signal 13 by reading out consecutively, from a memory havingthe level of the multi-level signal 13 stored therein, the level of themulti-level signal 13, which is corresponding to the combination of theinformation data 10 and the multi-level code sequence 12.

Further, in FIG. 2 and FIG. 3, the level of the multi-level signal 13 isrepresented as 8 levels, but the level of the multi-level signal 13 isnot limited to the representation. Further, the information amplitude isrepresented as three times or integer times of the step width of themulti-level signal 13, but the information amplitude is not limited tothe representation. The information amplitude may be any integer timesof the step width of the multi-level signal 13, or is not necessarilythe integer times thereof. Further, in FIG. 2 and FIG. 3, each of thelevels of the multi-level code sequence 12 is located so as to be at anapproximate center between each of the levels of the multi-level signal13, but each of the levels of the multi-level code sequence 12 is notlimited to such location. For example, each of the levels of themulti-level code sequence 12 is not necessarily at the approximatecenter between each of the levels of the multi-level signal 13, or maycoincide with each of the levels of the multi-level signal 13. Further,the above description is based on an assumption that the multi-levelcode sequence 12 and the information data 10 are identical in a changerate to each other and also in a synchronous relation, but the changerate of either thereof may be faster (or slower) than the change rate ofanother, and further, the both may be in an asynchronous relation.

Next, eavesdropping on the modulated signal 14 by a third party will bedescribed.

It is assumed that the third party, who is an eavesdropper, decodes themodulated signal 14 by using an apparatus having a configurationcorresponding to the data receiving apparatus 1201 held by a legitimatereceiving end or a further sophisticated data receiving apparatus(hereinafter referred to as an eavesdropper's data receiving apparatus).The eavesdropper's data receiving apparatus reproduces the multi-levelsignal 15 by demodulating the modulated signal 14. However, theeavesdropper's data receiving apparatus does not share key informationwith the data transmitting apparatus 1101, and thus, unlike the datareceiving apparatus 1201, the eavesdropper's data receiving apparatuscannot generate the multi-level code sequence 17, based on the keyinformation. Therefore, the eavesdropper's data receiving apparatuscannot perform binary determination of the multi-level signal 15 byusing the multi-level code sequence 17 as a reference.

As an action of the eavesdropping which may be possible under thesecircumstances, there is a method of simultaneously performingidentification of all levels of the multi-level signal 15 (generallyreferred to as an all-possible attack). That is, the eavesdropper's datareceiving apparatus performs simultaneous determination of themulti-level signal 15 by preparing thresholds corresponding to allpossible intervals between the respective signal levels which themulti-level signal 15 may take, and attempts extraction of correct keyinformation or information data by analyzing a result of thedetermination. For example, the eavesdropper's data receiving apparatussets all the levels c0/c1/c2/c3/c4/c5/c6 of the multi-level codesequence 12 shown in FIG. 2 as the thresholds, and performs themulti-level determination with respect to the multi-level signal 15,thereby attempting the extraction of the correct key information or theinformation data.

However, in an actual transmission system, a noise occurs due to variousfactors, and the noise is overlapped on the modulated signal 14, wherebythe respective levels of the multi-level signal 15 fluctuatestemporally/instantaneously as shown in FIG. 4. In this case, a SN ratio(a signal-to-noise intensity ratio) of a signal (the multi-level signal15), which is to be determined by the legitimate receiving end (that is,the data receiving apparatus 1201), is determined based on a ratio ofthe information amplitude of the multi-level signal 15 to the noiselevel. On the other hand, the SN ratio of the signal (the multi-levelsignal 15), which is to be determined by the eavesdropper's datareceiving apparatus, is determined based on a ratio of the step width ofthe multi-level signal 15 to the noise level.

Therefore, in the case where a condition of the noise level contained inthe signal to be determined is fixed, the SN ratio of the signal to bedetermined by the eavesdropper' data receiving apparatus is relativelysmaller than that by the data receiving apparatus 1201, and thus atransmitting feature (an error rate) of the eavesdropper's datareceiving apparatus is deteriorated. That is, the data communicationapparatus of the present invention utilizes this feature, and leads theall-possible attack by the third party using all the thresholds to anidentification error, thereby causing the eavesdropping to be difficult.Particularly, in the case where the respective step width of themulti-level signal 15 is set equal to or smaller than a noise amplitude(spread of a noise intensity distribution), the data communicationapparatus substantially disables the multi-level determination by thethird party.

As the noise to be overlapped on the signal to be determined (themulti-level signal 15 or the modulated signal 14), a thermal noise(Gaussian noise) contained in a space field or an electronic device,etc. may be used, in the case where an electromagnetic wave such as awireless signal is used as the modulated signal 14, and a photon numberdistribution (quantum noise) at the time of a photon being generated maybe used, in addition to the thermal noise, in the case where the opticalwave is used. Particularly, signal processing such as recording andreplication is not applicable to a signal including the quantum noise,and thus the step width of the multi-level signal 15 is set by using thequantum noise level as a reference, whereby it is possible to cause theeavesdropping by the third party to be difficult and to secure safety ofthe data communication.

As above described, according to the data communication apparatusaccording to the first embodiment of the present invention, when theinformation data to be transmitted is encoded as a multi-level signal,the interval between the signal levels of the multi-level signal is setappropriately with respect to a noise level so as to cause theeavesdropping by the third party to be difficult. With such setting,quality of the receiving signal at the time of the eavesdropping by thethird party is crucially deteriorated, and it is possible to provide afurther safe data communication apparatus which causesdecryption/decoding of the multi-level signal by the third party to bedifficult.

Second Embodiment

FIG. 5. is a block diagram showing a configuration of a datacommunication apparatus according to a second embodiment of the presentinvention. In FIG. 5, the data communication apparatus according to thesecond embodiment of the present invention has a configuration in whicha data transmitting apparatus 1102 and a data receiving apparatus 1202are connected to each other via a transmission line 110. The datatransmitting apparatus 1102 includes a multi-level encoding section 121and a modulator section 112. The multi-level encoding section 121includes a first multi-level code generation section 111 a, amulti-level processing section 111 b, a combining section 111 c, and anerror-correction encoding section 111 d. The data receiving apparatus1202 includes a demodulator section 211, and a multi-level decodingsection 222. The multi-level decoding section 222 includes a secondmulti-level code generation section 212 a, a multi-level identificationsection 212 b, and an error correction decoding section 212 c.

As shown in FIG. 5, the configuration of the data communicationapparatus according to the second embodiment illustrates in detailconfigurations of the first multi-level code generation section 111 aand the second multi-level code generation section 212 a, and isdifferent, compared to the data communication apparatus according to theabove-described first embodiment, in that the configuration includes thecombining section 111 c, the error-correction encoding section 111 d,and the error-correction decoding section 212 c. Hereinafter, componentswhich are the same as those of the above-described first embodiment willbe provided with common reference characters, and explanation thereof isomitted, and the data communication apparatus according to the secondembodiment will be described by mainly focusing on different components.

The first multi-level code generation section 111 a includes a firstrandom number sequence generation section 121 a and a first multi-levelconversion section 121 b. The first random number sequence generationsection 121 a inputs predetermined first key information 11 (step S601in FIG. 6), and generates a first binary random number sequence 31 ((a)of FIG. 9) by using the first key information 11. The first multi-levelconversion section 121 b converts, in accordance with a predeterminedencoding method, the first binary random number sequence 31 in amulti-level manner, and generates a multi-level code sequence 12 ((c) ofFIG. 8 and (b) of FIG. 9) in which a signal level changes so as to beapproximately random numbers (step S602 in FIG. 6).

Accompanying data 20 ((b) of FIG. 8) is inputted to the combiningsection 111 c (step S603 in FIG. 6). The accompanying data 20 is data tobe used to intentionally cause the information data 10 to bemisinterpreted, and, for example, a signal synchronizing with theinformation data 10 (an N frequency-dividing clock of information dataor a predetermined code synchronizing pattern, etc.) may be used. Thecombining section 111 c inputs the multi-level code sequence 12 and theaccompanying data 20, and in accordance with logic of the accompanyingdata 20, generates a multi-level code sequence (a combined multi-levelsignal) 21 ((d) of FIG. 8 and (c) of FIG. 9) in which a signal level ofthe multi-level code sequence 12 is converted with predeterminedfrequency (step S604 in FIG. 6).

The error correction encoding section 111 d inputs the information data10 (step S605 in FIG. 6), and outputs the information data 10 to themulti-level processing section 111 b after adding an error-correctioncode to the information data 10 (step S606 in FIG.6). The multi-levelprocessing section 111 b inputs the multi-level code sequence 21 and theinformation data 10 to which the error-correction code is added by theerror correction encoding section 111 d ((a) of FIG. 8), combines theboth signals in accordance with a predetermined procedure, and generatesa multi-level signal 13 having a level uniquely corresponding to acombination of levels of the combined both signals (step S607 in FIG.6). The modulator section 112 generates a modulated signal 14 in apredetermined modulation method, by using the multi-level signal 13 asoriginal data, and transmits the generated modulated signal 14 to thetransmission line 110 (step S608 in FIG. 6).

The demodulator section 211 inputs the modulated signal 14 (which is inthe predetermined modulation method and generated based on theinformation data 10 error encoded and the multi-level code sequence 21including the accompanying data 20) transmitted by the data transmittingapparatus 1102 via the transmission line 110 (step S701 in FIG. 7). Thedemodulator section 211 demodulates the modulated signal 14 andreproduces and outputs the multi-level signal 15 (step S702 in FIG. 7).The second multi-level code generation section 212 a includes a secondrandom number sequence generation section 222 a and a second multi-levelconversion section 222 b. The second random number sequence generationsection 222 a inputs second key information 16 which has the samecontent as the first key information 11 (step S703 in FIG. 7), andgenerates a second binary random number sequence 32 in accordance withthe second key information 16. The second multi-level conversion section222 b converts, in accordance with a predetermined encoding method, thesecond binary random number sequence 32, in a multi-level manner, andgenerates a multi-level code sequence 17 ((e) of FIG. 8) in which asignal level changes so as to be approximately random numbers (step S704in FIG.7).

The multi-level identification section 212 b identifies the multi-levelsignal 15 (binary determination) by using the multi-level code sequence17 as a threshold, and reproduces information data 22 ((f) of FIG. 8)(step S705 in FIG. 7). The error correction decoding section 212 cinputs the information data 22, and reproduces and outputs informationdata 18 ((g) of FIG. 8) by detecting error bits of the information data22 (or positions of the error bits: × marked positions in (f) of FIG. 8)and correcting errors of the error bits (step S706 in FIG. 7).

Here, the bit errors detected by the error correction decoding section212 c corresponds to difference between the multi-level code sequence 21and the multi-level code sequence 17, that is, a determination errorbased on logic of the accompanying data 20. Therefore, by detecting thepositions of the error bits, accompanying data 23 ((h) of FIG. 8) can bereproduced (step S707 in FIG.7).

Next, eavesdropping on a modulated signal by a third party will bedescribed. The third party does not share the first key information 11with the multi-level encoding section 121 of the data transmittingapparatus 1102, and thus performs simultaneous determination by means ofthe all-possible attack by preparing thresholds with respect to allintervals between respective signal levels possibly taken by ademodulated multi-level signal, and analyzing a result of thedetermination, thereby extracting correct key information or informationdata. Here, as a analyzing method of the key information by the thirdparty, there may be, for example, a method of decrypting the first keyinformation 11 by generating, based on a result of the determination, abinary random number sequence corresponding to the first binary randomnumber sequence 31 ((a) of FIG. 9) generated by the data transmittingapparatus 1102, and by obtaining, from the binary random numbersequence, consecutive bits (2 kbits) whose length is twice as long askey length k of the first key information 11. On the other hand, in thepresent invention, the signal level of the multi-level code sequence 12is converted depending on the accompanying data 20 in the datatransmitting apparatus 1102, whereby difference between the first binaryrandom number sequence 31 ((a) of FIG. 9) generated in the datatransmitting apparatus 1102 and the binary random number sequence ((e)of FIG. 9) generated based on the result of the determination areprovided. Accordingly, it is possible to prevent obtainment by the thirdparty of correct consecutive bits (2 k), and cause decryption of thefirst key information 11 to be difficult.

As above described, the data communication apparatus according to thesecond embodiment of the present invention adds the error-correctioncode to the information data 10 to be transmitted, and utilizes theaccompanying data 20 so as to cause the multi-level code sequence 21 inthe data transmitting apparatus 1102 to be in discord with themulti-level code sequence 17 in the data receiving apparatus 1202,thereby encrypting transmitting data and correcting errors in receivingdata occurring in the data receiving apparatus 1202. Accordingly, it ispossible to transmit/receive the information data 18 and theaccompanying data 23 simultaneously and realize a highly concealabledata communication apparatus.

Third Embodiment

FIG. 10 is a block diagram showing a configuration of a datacommunication apparatus according to a third embodiment of the presentinvention. In FIG. 10, the data communication apparatus according to thethird embodiment has a configuration in which a data transmittingapparatus 1102 and a data receiving apparatus 1203 are connected to eachother via a transmission line 110. The data transmitting apparatus 1102includes a multi-level encoding section 121 and a modulator section 112.The data receiving apparatus 1203 includes a demodulator section 211 anda multi-level decoding section 232. The multi-level decoding section 232includes a second multi-level code generation section 212 a, amulti-level identification section 212 b, an error correction decodingsection 212 c, and a synchronous extraction section 212 d.

As shown in FIG. 10, the configuration of the data communicationapparatus according to the third embodiment is different, compared tothe data communication apparatus according to the above-described secondembodiment, in that the configuration includes the synchronousextraction section 212 d. Hereinafter, with respect to such componentsthat are the same as those of the above-described second embodiment,description thereof will be omitted by providing common referencecharacters, and the data communication apparatus according to the thirdembodiment will be described by mainly focusing on different components.

The second random number sequence generation section 222 a generates asecond binary random number sequence 32 in accordance with second keyinformation 16 which has the same content as first key information 11.The second multi-level conversion section 222 b inputs a synchronoussignal 33, converts the second binary random number sequence 32, in amulti-level manner, into a multi-level code sequence 17, which is thensynchronized with the synchronous signal 33 and generated. Themulti-level identification section 212 b identifies the multi-levelsignal 15 (binary determination) by using the multi-level code sequence17 as a threshold, and reproduces information data 22. Theerror-correction decoding section 212 c inputs the information data 22,and simultaneously reproduces information data 18 and accompanying data23 by detecting error bits of the information data 22 and by correctingerrors of error bits. The synchronous extraction section 212 d inputsthe accompanying data 23, extracts the synchronous signal 33, which isin synchronization with the accompanying data 23, and outputs to thesecond multi-level conversion section 222 b.

In accordance with this action, in the case where, in the datatransmitting apparatus 1102, the information data 10 and theaccompanying data 20 are in synchronization with each other, it ispossible to obtain synchronization between the multi-level code sequence17 generated, based on the synchronous signal 33, by the secondmulti-level code generation section 212 a and the multi-level signal 15outputted from the demodulator section 211. Accordingly, the multi-levelsignal 15 can be synchronized with and identified by the multi-levelcode sequence 17.

As above described, the data communication apparatus according to thethird embodiment of the present invention adds error-correction code tothe information data 10, and causes the multi-level code sequences 21and 17, which are respectively in the data transmitting apparatus 1102and the data receiving apparatus 1203, to be in discord with each otherby using the accompanying data 20 synchronizing with the informationdata 10, and thereby encrypting transmitting data and correcting errorsin receiving data occurring in the data receiving apparatus 1202.Accordingly, it is possible to transmit/receive the information data 18and the accompanying data 23 simultaneously and realize a highlyconcealable data communication apparatus. Further, it is possible torealize a data communication apparatus of a simple configuration havinga synchronous system, by extracting the synchronous signal 33 from theaccompanying data 20 and by synchronizing and identifying themulti-level signal 15 using the multi-level code sequence 17 generatedbased on the synchronous signal 33.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A data transmitting apparatus for performing cipher communication,the data transmitting apparatus comprising: a multi-level codegeneration section for generating, by using predetermined keyinformation, a multi-level code sequence in which a signal level changesso as to be approximately random numbers; a combining section forcombining accompanying data to the multi-level code sequence withpredetermined frequency and generating a combined multi-level signal; anerror-correction encoding section for adding an error-correction code toinformation data, in accordance with predetermined error correctionencoding processing, and outputting error-correction encoded informationdata; a multi-level processing section for combining the combinedmulti-level signal and the error-correction encoded information data inaccordance with predetermined processing, and generating a multi-levelsignal having a level uniquely corresponding to a combination of levelsof both combined signals represented by the combined multi-level signaland the error-correction encoded information data; and a modulatorsection for generating a modulated signal, in a predetermined modulationmethod, based on the multi-level signal.
 2. The data transmittingapparatus according to claim 1, wherein the accompanying data is asynchronous signal which is synchronizing with the information data. 3.The data transmitting apparatus according to claim 1, wherein theaccompanying data is an N frequency-dividing clock of the informationdata.
 4. The data transmitting apparatus according to claim 1, whereinthe accompanying data has a predetermined code-synchronous pattern.
 5. Adata receiving apparatus for performing cipher communication, the datareceiving apparatus comprising: a demodulator section for demodulating amodulated signal generated, in a predetermined modulation method, basedon error-correction encoded information data and a combined multi-levelsignal having accompanying data included therein, and outputting amulti-level signal obtained by the demodulation; a multi-level codegeneration section for generating, by using predetermined keyinformation, a multi-level code sequence in which a signal level changesso as to be approximately random numbers; a multi-level identificationsection for identifying the multi-level signal in accordance with themulti-level code sequence, and outputting data which is identified andreproduced; and an error-correction decoding section for detecting, fromthe data reproduced by the multi-level identification section,difference between the combined multi-level signal and the multi-levelcode sequence, in accordance with predetermined error-correctiondecoding processing, and outputting a result of the detection as theaccompanying data, and also outputting information which iserror-correction decoded as the information data.
 6. A data receivingapparatus for performing cipher communication, the data receivingapparatus comprising: a demodulator section for demodulating a modulatedsignal, in a predetermined modulation method, generated based onerror-correction encoded information data and a combined multi-levelsignal having accompanying data included therein, and outputting amulti-level signal obtained by the demodulation; a multi-level codegeneration section for generating, by using predetermined keyinformation and a synchronous signal, a multi-level code sequence inwhich a signal level changes so as to be approximately random numbers; amulti-level identification section for identifying the multi-levelsignal in accordance with the multi-level code sequence, and outputtingdata which is identified and reproduced; an error-correction decodingsection for detecting, from the data reproduced by the multi-levelidentification section, difference between the combined multi-levelsignal and the multi-level code sequence, in accordance withpredetermined error-correction decoding processing, and outputting aresult of the detection as the accompanying data, and also outputtinginformation which is error-correction decoded as the information data;and a synchronous extraction section for inputting the accompanyingdata, extracting the synchronous signal synchronizing with theaccompanying data in accordance with a predetermined procedure, andoutputting the extracted synchronous signal to the multi-level codegeneration section.
 7. The data receiving apparatus according to claim6, wherein the synchronous extraction section further inputs apredetermined code-synchronous pattern, and extracts the synchronoussignal synchronizing with the accompanying data in accordance with apredetermined procedure.
 8. A data transmitting method for performingcipher communication, the data transmitting method comprising: a step ofgenerating, by using predetermined key information, a multi-level codesequence in which a signal level changes so as to be approximatelyrandom numbers; a step of combining accompanying data to the multi-levelcode sequence with predetermined frequency and generating a combinedmulti-level signal; a step of adding an error-correction code toinformation data, in accordance with predetermined error-correctionencoding processing, and outputting the error-correction encodedinformation data; a step of combining the combined multi-level signaland the error-correction encoded information data, in accordance withpredetermined processing, and generating a multi-level signal having alevel uniquely corresponding to a combination of levels of both signalsrepresented by the combined multi-level signal and the error-correctionencoded information data; and a step of generating a modulated signal,in a predetermined modulation method, based on the multi-level signal.9. A data receiving method for performing cipher communication, the datareceiving method comprising: a step of demodulating a modulated signal,in a predetermined modulation method, generated based onerror-correction encoded information data and a combined multi-levelsignal having accompanying data included therein, and outputting amulti-level signal obtained by the demodulation; a step of generating,by using predetermined key information, a multi-level code sequence inwhich a signal level changes so as to be approximate random numbers; astep of identifying the multi-level signal based on the multi-level codesequence and outputting data which is identified and reproduced; and astep of detecting, from the data reproduced by the multi-levelidentification section, difference between the combined multi-levelsignal and the multi-level code sequence, in accordance withpredetermined error-correction decoding processing and outputting aresult of the detection as the accompanying data, and also outputtinginformation which is error-correction decoded as the information data.10. A data receiving method for performing cipher communication, thedata receiving method comprising: a step of demodulating a modulatedsignal, in a predetermined modulation method, generated based onerror-correction encoded information data and a combined multi-levelsignal having accompanying data included therein, and outputting amulti-level signal obtained by the demodulation; a step of generating,by using predetermined key information and a synchronous signal, amulti-level code sequence in which a signal level changes so as to beapproximate random numbers; a step of identifying the multi-level signalin accordance with the multi-level code sequence and outputting datawhich is identified and reproduced; a step of detecting, from the datareproduced by the multi-level identification section, difference betweenthe combined multi-level signal and the multi-level code sequence, inaccordance with predetermined error-correction decoding processing, andoutputting a result of the detection as the accompanying data, and alsooutputting information which is error-correction decoded as theinformation data; and a step of inputting the accompanying data,extracting the synchronous signal synchronizing with the accompanyingdata in accordance with a predetermined procedure, and outputting theextracted synchronous signal to the multi-level code generation section.